The present invention relates to thermal spray technology. More particularly, the present invention relates to single-wire arc spray apparatus and methods for producing a focused, narrow beam spray.
Thermal spray processes are known for use in applying coatings to a variety of substrates such as metals, ceramics, and plastics. Moreover, such spray processes are advantageous for use in the fabrication of freestanding, three dimensional structures via the build-up of coating layers.
Generally speaking, thermal spray devices produce spray material in accordance with one of three operating principles: combustion, plasma, or wire arc. For many coating applications, wire arc spray has emerged as the technique of choice. This is primarily, although not exclusively, attributable to the ability of wire arc spray devices to yield a quality coating with the use of relatively inexpensive spraying equipment and materials. In addition, wire arc spraying has low power requirements, is energy efficient, and can be used to coat substrates having relatively low thermal limits.
Conventional wire arc spray devices use a gun having two converging and consumable wire electrodes. An arc is formed between the electrodes, resulting in molten material at the electrode tips which is stripped away and atomized by a carrier gas. The atomized coating material is then directed to a substrate for spray coating. A discussion of wire arc spraying may be found in; Optical Diagnostics and Modeling of Gas and Droplet Flow in Wire Arc Spraying, Kelkar et al., Proceedings of the 15th International Thermal Spray Conference, pp. 329-334 (1998); and Thermal Spray: New Technology is its Lifeblood, Irving, Welding Journal, Vol. 77, no. 3, pp. 38-45 (1998).
In addition to twin-wire arc spray devices, some thermal spray systems produce a thermal spray with the use of a single-wire wherein the arc is typically formed with the spray nozzle. For instance, see Recent Developments in Arc Spraying, Steffens et al., IEEE Transactions on Plasma Science, Vol. 18, no. 6, pp. 974-979 (1990); and U.S. Pat. No. 3,064,114 (Cresswell et al.).
While these wire arc spray processes are effective, problems remain. For instance, devices that arc to the nozzle may result in erratic arc attachment. This may lead to inconsistent spray characteristics and possibly premature nozzle clogging.
Moreover, spray output generated by many nozzle arcing devices as well as by various twin-wire systems may rapidly diverge upon exiting the spray nozzle. In some devices, angular spray divergence of 20 degrees or more is not uncommon. In twin wire systems, divergence can at least partially be attributed to the different polarity of the two wires.
Spray divergence is undesirable for several reasons. For instance, divergence results in decreased flux density of the spray material as the spray expands. As flux density decreases, some degree of droplet solidification may occur during spraying, resulting in a porous and nonuniform coating. Divergence may also produce a spray coating having a nonuniform thickness, e.g., a coating that is noticeably thicker near the center of the spray pattern and thin and/or uneven near the outer edges. Still further, divergence of the sprayed material may also result in excessive dust and overspray (spray outside the intended target spray area). For at least these reasons, masking of the substrate, multiple spray passes, and subsequent surface finishing are often required to achieve coatings having a uniform thickness.
As a result of these issues, systems able to produce a more focused thermal spray pattern have emerged. For example, U.S. Pat. No. 4,370,538 (Browning) discloses a high velocity dual stream flame spraying system. While effective for its intended purpose, the ""538 invention may not include benefits (e.g., low cost equipment, usable with thermally sensitive substrates) available with some wire arc spraying systems.
Other patents, see e.g., U.S. Pat. Nos. 4,492,337 (Harrington et al.) and 5,191,186 (Crapo III et al.), on the other hand, are directed to improvements to twin-wire spraying apparatus that yield higher quality coatings. While effective, these apparatus still utilize two consumable electrodes of opposite polarity. As a result, potential spray instabilities due to irregularities inherent in the process of simultaneously feeding two wires are possible.
The present invention is directed to single-wire arc spray apparatus and methods of use that yield a narrow beam spray, and thus a controlled width spray pattern, having highly defined edges. Apparatus and methods of the present invention furthermore produce such advantageous spray patterns without the problems commonly associated with other wire arc devices.
In one embodiment, a liquid material droplet generator is provided. The generator includes a gas nozzle having a nozzle entrance, a nozzle exit, and a nozzle bore where the nozzle bore defines a nozzle axis. A first consumable electrode is also included and is positionable within the nozzle bore of the gas nozzle. A second non-consumable electrode positionable outside the gas nozzle proximate the nozzle exit is also provided.
In another embodiment, a liquid material droplet generator is provided and includes means for forming a gas jet, wherein the means for forming the gas jet comprises a passageway having an exit. The generator further includes means for delivering a consumable feedstock to the exit and along an axis of the passageway, and means for establishing a heat zone outside of the passageway and adjacent the exit. The means for establishing the heat zone is adapted to melt at least a portion of the consumable feedstock to form liquid droplets.
In another embodiment, a liquid material droplet generating system is provided. The system includes a single-wire arc spray apparatus having a gas nozzle with a nozzle entrance, a nozzle exit, and a nozzle bore, the nozzle bore defining a nozzle axis. The spray apparatus further includes a first consumable electrode positionable within the nozzle bore, wherein the first consumable electrode has a first electrode axis, and a second non-consumable electrode positionable outside the gas nozzle proximate the nozzle exit. The system also includes a power supply apparatus adapted to connect to at least the first consumable electrode and the second non-consumable electrode, and a feeding apparatus adapted to feed the first consumable electrode through the nozzle bore. A controller adapted to control one or more of the power supply apparatus and the feeding apparatus may also be provided.
A method of generating a narrow beam thermal spray of liquid droplets is also provided. The method includes providing a gas nozzle having a nozzle entrance, a nozzle exit, and a nozzle bore, where the nozzle bore defines a nozzle axis. The method also includes positioning a first consumable electrode within the nozzle bore of the gas nozzle and positioning a second non-consumable electrode outside of the gas nozzle proximate the nozzle exit. An electrical arc may be formed outside of the gas nozzle proximate the nozzle exit, where the electrical arc is formed between a terminal end of the first consumable electrode and a portion of the second non-consumable electrode.
In yet another embodiment of the present invention, a method for forming a high density microstructure is provided. The method may include providing a gas nozzle having a nozzle entrance, a nozzle exit, and a nozzle bore, where the nozzle bore defines a nozzle axis. The method may also include positioning a first consumable electrode within the nozzle bore of the gas nozzle and positioning a second electrode outside of the gas nozzle and proximate the nozzle exit. A first arc gas may be accelerated through the gas nozzle to form a gas jet at the nozzle exit. An electrical arc may be formed outside of the gas nozzle proximate the nozzle exit, where the electrical arc is formed between a terminal end of the first consumable electrode and a portion of the second electrode. The electrical arc causes a portion of the first consumable electrode to melt and form droplets near a center of the gas jet, forming a narrow beam thermal spray. The method may also include depositing the droplets on a substrate surface to form a coating, where the coating is defined by substantially indiscernible boundaries between the droplets that form the coating.
The above summary of the invention is not intended to describe each embodiment or every implementation of the present invention. Rather, a more complete understanding of the invention will become apparent and appreciated by reference to the following detailed description and claims in view of the accompanying drawings.